1. Field of the Disclosure
This disclosure relates generally to apparatus and methods for determining a distance to a pre-existing wellbore and controlling drilling operations based on the determination.
2. Background of the Art
In the process of drilling wells for hydrocarbon production, it is commonly necessary to drill a second well in a predetermined relationship to an existing well. An example of this may be when a blowout occurred in the existing well; two approaches may be taken to control the blowout. One method is to use explosives at the surface and snuff out the fire in the burning well. This procedure is fraught with danger and requires prompt control of hydrocarbons flow in the well. The second method is to drill a second borehole to intersect the blowout well and pump drilling mud into the blowout well. This is not a trivial matter. An error of half a degree can result in a deviation of close to 90 feet at a depth of 10,000 feet. A typical borehole is about 12 inches in diameter, a miniscule target compared to the potential error zone.
Another situation in which accurate drilling is required is in secondary recovery operations. For various reasons, such as low formation pressure or high viscosity of hydrocarbons in the reservoir, production under natural conditions of hydrocarbons may be at uneconomically low rates. In such cases, a second borehole is drilled to be substantially parallel to the pre-existing borehole. Fluid such as water, CO2 is then injected into the formation from the second borehole and the injected fluid drives the hydrocarbons in the formation towards the producing borehole where it may be recovered.
In 1970, Shell Oil Co.'s Cox 1, a 22,000-ft Smackover exploratory well, blew out near Piney Woods, Miss. This challenge led to the first direct intersection of a blowout tubular using an acoustic detection method. Wireline instruments were developed to detect proximity of a tubular by measuring distance and direction from the relief well to the blowout casing using the noise from the flowing gas in the blowout well. More recently, electromagnetic methods have been used to determine the distance to the cased preexisting well.
The electromagnetic techniques fall into two (2) categories. In the first category, referred to as active ranging, a source of AC magnetic field and a magnetic sensor are placed in different wells. The source can be a solenoid placed in the production well or an electric current injected in the production well casing. The magnetic field produced by the current in the casing is measured in the drilling well. The active ranging approach can probably offer a good accuracy of measurements, but suffers from the drawback that access to the pre-existing well is required.
In the second category are passive ranging techniques that do not require access to the pre-existing well while drilling the second well. The techniques normally utilize a relatively strong magnetism induced in the casing of the pre-existing well by the Earth's magnetic field. The signal due directly to the earth's magnetic field is a problem, limiting the accuracy of this measurement. Residual magnetism of the casing introduces additional uncertainties.
The present disclosure discloses apparatus and methods for determining distance from a pre-existing wellbore in which access to the pre-existing well is not required and the effects of the direct earth's magnetic field are minimized.